This invention relates to electronic ballasts for gas discharge lamps and, in particular, to an electronic ballast having a height equal to or less than one inch (2.5 cm.), which is approximately equal to the diameter of a tubular fluorescent lamp or to the smallest dimension of a folded, compact lamp.
A fluorescent lamp is a non-linear load, i.e. the current through the lamp is not proportional to the voltage across the lamp and the current will increase rapidly unless there is a ballast in series with the lamp to limit current. A "magnetic" ballast is an inductor in series with the lamp for limiting current through the lamp. The inductor includes many turns of wire wound on a laminated iron core and magnetic ballasts of the prior art are physically large and heavy, often accounting for more than half the weight of a fixture including the lamps.
A characteristic of magnetic ballasts is poor power factor. Power factor indicates whether or not a load in an AC circuit is equivalent to a pure resistance (a power factor of 1.0). If the power factor is less than 1.0, then the current and voltage are out of phase, causing increased power consumption. Regulations in many states require a high minimum power factor, e.g. 0.85, as part of energy conservation measures and the effect of these regulations is to require the use of "electronic" ballasts rather than magnetic ballasts.
Another characteristic of electronic ballasts is poor (high) harmonic distortion. Harmonic distortion is a measure of the harmonic content of an AC signal. As originally shown by the French mathematician Fourier, a square wave of a given frequency can be approximated by combining the fundamental and odd harmonics of a sinusoidal wave having the same frequency as the square wave. The waveform of the AC signal in a power line is sinusoidal and has a frequency of 60 hz. in the U.S.A. If the current drawn by a ballast is essentially in the form of square wave pulses, then the ballast is said to exhibit harmonic distortion because of the odd harmonics of 60 hz. in the current through the ballast.
One can improve the power factor of a ballast and still exhibit a large harmonic distortion. For example, a 60 hz. square wave signal can appear to have a power factor as high as 0.95. However, harmonic distortion is 30% or more. Harmonic distortion is a problem for the three-phase circuits typically found in commercial buildings because odd harmonics do not cancel out on the neutral line of a three phase system. Since the odd harmonics do not cancel out, power consumption is increased.
An electronic ballast typically includes a converter for changing the AC from a power line to direct current (DC) and an inverter for changing the DC to high frequency AC. Converting from AC to DC is usually done with a full wave or bridge rectifier. A filter capacitor on the output of the rectifier stores energy for powering the inverter. Some ballasts include a "boost" circuit to improve power factor or to increase the voltage on the filter capacitor from approximately 140 volts to 300 volts or higher (from a 120 volt AC input). The inverter changes the DC to high frequency AC at 140-300 volts for powering one or more fluorescent lamps.
Because electronic ballasts operate at a higher frequency than a power line (e.g. 30 khz compared to 50/60 hz), the "magnetics" in an electronic ballast are much smaller than the inductor in a magnetic ballast. (As used herein, "inductor" means a device having a single winding on a core, "transformer" means a device having more than one winding on a common core, and "magnetic" used as a noun is generic for either device. The core can be air, powdered ferrite, laminated iron sheets, or other material.)
Electronic ballasts are lighter than magnetic ballasts but have the disadvantage of including many more components, increasing the cost of the ballast. The magnetics in an electronic ballast contribute significantly to the size, weight, and the cost of the ballast and set a lower limit to the size of the ballast. While the magnetics can be made in almost any shape, it can be shown that the most efficient and least expensive magnetics are essentially cubic. The dimensions of the magnetics therefore set a lower limit to the smallest dimension of a ballast, herein referred to as the height or thickness of the ballast.
Boost circuits and inverters include at least one magnetic. The different functions of these magnetics, power factor correction and output, can be combined but the resulting magnetic is larger than separate magnetics for performing these functions.
The efficiency of a ballast is the power consumed by a lamp divided by the total power supplied to the lamp and the ballast. The efficiency of the magnetics in a ballast is a part of the overall efficiency, which includes transistor losses. While efficiency cannot equal one hundred percent, an efficiency of over ninety percent is desirable and attainable. A problem with ballasts of the prior art is obtaining high efficiency while using small magnetics.
An electronic ballast is not intended to be operated without a lamp. Unfortunately, it is common practice to change a lamp while power is applied to the lamp. If a lamp is not connected to the ballast, or if a lamp is defective, then the voltage on the sockets for the lamp can exceed 300 volts. This creates a hazardous situation for anyone who may come into contact with a socket.
One solution to this problem is to use a transformer as the output magnetic, thereby isolating the sockets from ground and from the fixture for the lamp. An output transformer is larger than the diameter of a fluorescent lamp even for electronic ballasts operating at high frequency. It is extremely desirable to provide a ballast that has a height less than or equal to one inch (2.5 cm.), which is approximately the diameter of the very efficient T8 lamp or the smallest dimension of a folded, compact lamp. A small height provides a lighting designer with great flexibility in locating lighting in a room.
A ballast without an output transformer is known as having a direct coupled output. Such ballasts require additional circuitry to sense fault conditions, such as a missing or defective lamp, and to shut off the ballast. A problem with fault detection circuitry is the power consumed when the lamp is operating normally, i.e. adding fault detection circuitry can decrease the efficiency of a ballast.
In view of the foregoing, it is therefore an object of the invention to provide a low cost, light weight, electronic ballast having a height of one inch (2.5 cm.) or less.
Another object of the invention is to provide an electronic ballast having high power factor, efficient magnetics, safe operation without a lamp, and small height.
A further object of the invention is to provide an electronic ballast which is efficient, fault tolerant, and has a height equal to or less than the smallest dimension of a tubular fluorescent lamp or a folded, compact lamp.
Another object of the invention is to provide an efficient electronic ballast in which the components dissipate little power and operate at low temperature for high reliability.
Another object of the invention is to provide an electronic ballast having an efficiency of greater than ninety percent, a power factor greater than ninety percent, a total harmonic distortion less than ten percent, and a height of less than one inch.
SUMMARY OF THE INVENTION
The foregoing objects are achieved in the invention in which an electronic ballast has a high voltage portion and a low voltage portion wherein the number of components in the high voltage portion are minimized. The high voltage portion includes a converter, having a variable frequency boost circuit, and a driven half-bridge inverter having a series resonant, direct coupled, parallel output. The low voltage portion of the ballast includes a control circuit and fault detectors for shutting off the boost circuit and the inverter circuit. The fault detectors consume very little power when the ballast and lamp are functioning normally. Separate magnetics are used for boost, inverter, and output. Each magnetic is essentially cubic in shape and carries as little current as possible to minimize the size of the magnetic and to minimize the height of the ballast.